(1) Field of the Invention
The present invention relates to a brushless three-phase synchronous generator. More particularly, the present invention relates to a circuit construction of the rotor field windings for use in the brushless three-phase synchronous generator, which is so devised that an improvement is made on waveform of the current flowing in the rotor field windings induced based on the odd-order higher harmonic magnetic fluxes generated by the stator excitation windings and that an enhancement is achieved on the excitation efficiency.
(2) Description of the Related Art
First, a conventional brushless three-phase synchronous generator is explained with reference to FIGS. 1 and 2. FIG. 1 shows a circuit diagram of the conventional brushless three-phase synchronous generator 40. At a stator side 41, there are provided three-phase four-pole primary generating windings 42, stator excitation windings 43 whose number of poles is 12, odd-number times the number of poles of the primary generating windings 42, and a DC power source 44 which is formed by a variable resistor 48 and a plurality of diodes 49. On the other hand, at a rotor side 45, there are provided a plurality of rotor field windings 46 wound in a full-pitch concentrated winding manner, whose number of poles is the same as that of the primary generating windings 42 of the rotor 41. Each of the rotor field windings is short-circuited by a diode 47.
FIG. 2 shows a further detailed construction of the rotor 45. The plurality of field windings W.sub.f1 -W.sub.f6 (generally shown by numeral 46 in FIG. 1) are wound on a cylindrical field system 20 so as to form a four-pole construction, and the field windings W.sub.f1 -W.sub.f6 are respectively short-circuited by the corresponding diodes D1-D6 (generally shown by numeral 47 in FIG. 1).
Operation of the above brushless three-phase synchronous generator 40 under a non-load state is as follows. Upon the initial rotation of the stator 45, there is induced an electromotive force in the primary generating windings 42 of the stator due to the residual magnetism of the rotor iron. This electromotive force causes the alternating current (AC) to flow in the primary generating windings 42 through the DC power source 44 and also the stator excitation windings 43. Based on this AC current, there are produced armature reaction magnetic fields around the primary generating windings 42. DC currents flowing in the stator excitation windings 43 due to the function of the DC power source 44 produce a static magnetic field around the stator excitation windings 43. The overlapped magnetic fields formed by the armature reaction magnetic field and the static magnetic field cause the electromotive forces to be induced in the plurality of respective field windings 46 of the rotor 45, which field windings are magnetically coupled with all the odd-order spatial higher harmonic components of the armature reaction magnetic fields and the static magnetic fields. The electromotive forces thus induced in the field windings are respectively half-wave rectified by the corresponding diodes D1-D6, and the DC components thus obtained function to increase the field magnetic fluxes of the rotor. As the field magnetic fluxes increase, the electromotive forces induced in the primary generating windings 42 of the stator also increase. In this way, the generated voltage gradually goes up and finally reaches the self-establish maximum voltage at the non-load state.
Here, as the rotor field windings 46 (W.sub.f1 -W.sub.f6) are wound in a full-pitch concentrated winding manner so as to have the same number of poles as that of the primary generating windings 42 of the stator and further they are respectively short-circuited by the corresponding diodes 47 (D1-D6), they react with all the odd-order spatial higher harmonic components and, therefore, function to increase the field magnetic fluxes of the rotor field windings 46.
In the case where the balanced resistor loads or balanced inductive loads are connected to the output terminals X, Y and Z of the generator 40, load currents flow in the primary generating windings 42 and, thus, the armature reaction magnetic fields produced around the primary generating windings 42 are increased due to the load currents. Therefore, the odd-order spatial higher harmonic magnetic fields of the armature reaction magnetic fields increase in proportion to the increase in the load currents. The increase in the odd-order spatial higher harmonic magnetic fields causes the increase in the electromotive forces induced in the rotor field windings 46 accordingly and, therefore, the electromotive forces induced in the primary generating windings increase due to the increase in the primary magnetic fluxes of the rotor. For this reason, since the impedance voltage drop in the primary generating windings 42 caused by the load currents is compensated by the increase in the induced electromotive forces therein, the capacity required to the DC power source 44 can be reduced by this extent of compensation.
Because the current to flow in the rotor field windings which are magnetically coupled with the stator excitation windings is determined by the ampere-turn law and the field magnetomotive forces are produced based on the DC components obtained by half-wave rectification of the current flowing in the field windings, in the case where the number of the field windings is small, the amplification factor, that is, the ratio of the magnetomotive force of the field windings with respect to that of the stator excitation windings is small and, thus, the capacitor required to the automatic voltage regulator (AVR) inevitably becomes large. For this reason, there has been a demand for developing an improved generator having high excitation efficiency in which the amplification factor of the magnetomotive forces is large and by which the capacitor of the automatic voltage regulator can be reduced.
Since the field windings are wound on the periphery of the rotor with evenly distributed, some of the field windings do not effectively produce the field magnetic flux, resulting in lowering of the generator efficiency. Thus, the improvement in the generator efficiency has been a long standing demand.
Further, in the case where unbalanced three-phase loads or single-phase loads are connected to the generator, though there occurs a compensation operation, electromotive forces are induced in the rotor field windings due to the armature reaction magnetic fields of the opposite-phase and the alternating current of the double frequency is caused to flow in the field windings because the opposite-phase armature reaction magnetic fields interlink with the field windings at a speed twice the synchronous speed. These phenomena stem from the fact that rotor field windings function also as damper windings. Where the number of field windings is large, the function to increase the magnetic fields becomes large and, thus, there is a likelihood of the output voltage becoming excessively high.